1. Field of the Invention
The present invention relates to an information recording technique on an optical disc.
2. Description of Related Art
Information recording on a recordable-type optical disc on which information is additionally recordable and rewritable is performed by driving a laser light source by a pulse signal corresponding to recording data, generating a recording laser light and irradiating it on an information recording surface of the optical disc. As a laser light source, a semiconductor laser such as a laser diode is utilized. Power of the recording laser light is controlled by controlling current quantity which is supplied to the laser diode by a driving circuit such as a laser driver. The laser driver switches plural current sources whose current quantity is preset according to a driving pulse signal corresponding to information to be recorded, and drives the laser diode. The driving pulse signal controls the laser emitting power from the laser diode, and recording pits (recording marks) corresponding to data to be recorded are formed on the optical disc.
Hence, a noise characteristic of the semiconductor laser largely affects recording quality. For example, a portion of an outputting light from the semiconductor laser returns to the semiconductor laser by a reflected light from a recording medium and other causes, a jump in a longitudinal mode, a so-called “mode hopping”, occurs, and laser output is varied. As a result, a noise occurs. This kind of noise is particularly problematic in the field of the semiconductor laser. As a method of reducing the noise of such the semiconductor laser, there is known a so-called high frequency signal superimposing method which superimposes a high frequency signal on a recording pulse signal in recording the information on the optical disc.
In adopting the high frequency signal superimposing method, there are suggested two methods, i.e., a method of varying a frequency and an amplitude of a superimposed high frequency signal according to an operation mode of the optical disc recording apparatus, which is disclosed in Japanese Patent Application Laid-open under No. 2000-149302, and a method of controlling a superimposing timing of the high frequency signal, which is disclosed in Japanese Patent Application Laid-open under No. 2002-230813.
Recently, there is a strong demand for high-speed recording. For the purpose of realizing the high-speed recording, the recording pulse signal tends to become high-speed, i.e., the frequency of the recording pulse signal tends to become higher. As a result, there occurs a problem that the frequencies of the superimposed high frequency signal and the recording pulse signal become closer to each other, and interference occurs between them. Specially, at the rise-up and fall-down portions of the recording pulse signal, an effect which the high frequency signal gives the recording pulse signal becomes large, and the recording quality may be deteriorated.
As described in the above-mentioned Japanese Patent Applications, in the conventional high frequency signal superimposing method, since the high frequency signal is generated by an analog circuit, the high frequency signal does not have correlation with the recording pulse signal. Thus, it is difficult to perform an optimum control of the frequency and an superimposing quantity of the high frequency signal according to a power of the laser light during recording the information, such as a write power, an erase power, and a bias power. Also, since the high frequency signal is not synchronous with the recording pulse signal, the phase relation between the high frequency signal and the recording pulse signal cannot be controlled. Therefore, a driving pulse signal for driving the semiconductor laser becomes unstable by the effect of the high frequency signal, and the recording quality of the information can be deteriorated.